This is an old revision of the document!
The method below is a good way but you can also adhere a glass bead thermistor to the underside with some Kapton tape and then maybe cover with a bit of foil tape.
Monoprice Select Mini Heated Bed Thermistor Replacement - by Mark J. Blair, NF6X
http://www.nf6x.net/2016/07/monoprice-select-mini-heated-bed-thermistor-replacement